Asynchronous communication
Designing the 'interface' is designing the messages that asynchronously flow between services.
Messages as .NET types
Messages with NServiceBus are .net classes or interfaces that are (de)serialized from/to xml, json or other serializer.
http://docs.particular.net/nservicebus/messaging/messages-as-interfaces
This means that we expect a xml or json message to be deserialized to the corresponding .net type with the configured serializer.
If you model messages as XML/JSON and then can have that generate a type safe class representation then you are fine. If not, then you miss out on the programming model that NServiceBus tries to apply.
Native integration without NServiceBus
However, messages on the wire are XML/JSON or any format that you choose which makes it easier to natively integrate via the underlying transport like MSMQ, RabbitMQ, SQL, Azure Service Bus, Azure Storage Queues or any of the community supported transports if the receiving or sending side is not a .NET NServiceBus process.
However, you will that need to implement your own serialization, transaction management, pubsub and process pipeline.
Integration patterns
If you want to integrate with existing applications or external systems then a good book to get familiar with common integration patterns is:
http://www.enterpriseintegrationpatterns.com/
Data transformation and protocol bridging
Related to building integrations I think a good read would be :
http://udidahan.com/2011/04/08/integration-how-and-where/
The blog post talks about data transformation and protocol bridging. These are not native features that NServiceBus provides.
NServiceBus integration channels
It is fairly common that you have NServiceBus integration services that would take care of the transformation and communication for a specific integration channel. Such channels could very well have a different (public) schema from how the data is stored and/or communicated within a collection of independent components that represent a SOA service or system and that use NServiceBus for communication.
Other resources
This might help you wrap your existing systems and expose them as messages based systems which can send/receive asynchronous messages.
You might find relevant videos here: http://particular.net/videos-and-presentations